High temperature ferrous alloy containing nickel,chromium and aluminum



United States atent O 3,540,881 HIGH TEMPERATURE FERROUS ALLOY CONTAINING NICKEL, CHROMIUM AND ALUMINUM Carol Henry White, Burley Gate, John Woolridge Eggar,

Hereford, and Harry Gayter, Hampton Bishop, England, assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Sept. 28, 1967, Ser. No. 671,244

Claims priority, application Great Britain, Oct. 3, 1966,

44,114/ 66 Int. Cl. C22c 39/02, 39/20 U.S. Cl. 75-124 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to nickel-chromium-iron alloys particularly suitable for wrought rotor discs for gas turbine engines and for other wrought articles and parts that are subjected in use to high stress at temperatures up to 700 C., for example in the range of 550-650 C.

The different properties required of a rotor disc material are manifold and complex and, to a large extent, conflicting. Of particular significance is the large variation of temperature occurring radially between the centre or hub and the periphery or rim of the disc. This temperature gradient is accompanied by a stress gradient in the opposite sense so that the highest stress occurs in the low-temperature region near the hub and vice versa. A rotor disc material must therefore have a high creep strength up to the highest temperature to which it is exposed to ensure freedom from distortion by creep in service, particularly at the rim, and a high proof stress and ultimate tensile strength at more moderate temperatures to ensure that the high hub stresses do not lead to distortion or fracture on loading. It must have adequate tensile ductility and must not be notch-sensitive at temperatures corresponding to that at which the rim, with its fir-tree recesses for the turbine blades, operates. Furthermore, the need to produce a large and relatively complex shape requires that the alloy shall be hot-workable. Finally it is desirable that the cost of the alloy should be as low as possible.

Typical of the low-cost alloys hitherto used for rotor discs is the nickel-chromium-iron Alloy 901, the nominal composition of which, in weight percent, is carbon 0.05, chromium 12.5, nickel 42.5, molybdenum 5.7, titanium 2.8, aluminium 0.2, boron 0.015, balance iron and impurities. While this alloy has a very satisfactory combination of properties, the progressive increase in the designed operating temperatures and stresses of gasturbine engines calls for a material having a substantially higher proof stress. While this can be obtained by the use of nickel-chromium-base alloys containing high contents of such alloying elements as molybdenum and nibium, these alloys have the serious disadvantages of being very diflicult to hot work and extremely difficult to machine.

According to the invention, alloys that have substantially improved proof stress and yet are still readily hotworkable and machinable contain from 0.02 to 0.1%

Patented Nov. 17, 1970 carbon, from 11 to 16 chromium, from 4 to 7% molybdenum, from 0.3 to 0.8% niobium, from 2.0 to 3.5% titanium, from 0.25 to 0.75% aluminium, from 35 to 45% nickel, from 0.003 to 0.02% boron, and from 0 to 0.1% zirconium, the balance apart from impurities, being iron. The main impurities commonly present in nickel-chromium-iron alloys are silicon and manganese, and the amounts of these elements should not exceed 0.5% each. Preferably the silican content does not exceed 0.3%. Tantalum is commonly present as an impurity in the commercially available forms of niobium, and niobium may be replaced by tantalum on an atom for atom basis up to a maximum tantalum content of 0.1%, such amounts of tantalum being regarded as part of the niobium content.

It is important that the content of each of the constituents of the alloys lies Within the ranges set forth. With less than 0.02% carbon the alloys are notch-sentitive, their life in stress-rupture tests being greatly reduced by the presence of notches, and preferably the carbon content is at least 0.03%. Too much carbon, on the other hand, leads to the formation of excessive amounts of carbides, which tend to segregate and give rise to directionality effects in the tensile and stressrupture properties of the wrought material. Hence the carbon content must not exceed 0.1% and is preferably not more than 0.08% and most advantageously not more than 0.06%.

At least 11% chromium is required for adequate resistance to oxidation at the operating temperature, but more than 16% makes the alloys liable to embrittlement on prolonged heating.

Molybdenum contributes to the stress-rupture strength of the alloys, and at least 4% is required for this purpose. Raising the molybdenum content above 7%, however, makes the alloy very difiicult to work and also susceptible to enbrittlement, and preferably the molybdenum content is from 5.0 to 6.5%.

The amount of niobium is particularly important. At least 0.3% is required for adequate proof stress, but increasing the niobium content above 0.8% is accompanied by a rapid fall in tensile ductility and by susceptibility to embrittlement on prolonged heating. Preferably the niobium content is from 0.4 to 0.7%.

Titanium hardens the alloys and contributes to their stress-rupture strength, but increasing the titanium content reduces the ductility, so not more than 3.5% titanium may be present.

At least 0.2% of aluminium is required to avoid the risk of embrittlement. Surprisingly, however, increasing the aluminum content, while increasing the tensile ductility of the alloys, is found to lower their proof stress. For this reason the aluminium content must not exceed 0.75 and for the best combination of proof stress and ductility it is preferably from 0.3 to 0.5%.

The alloys must contain at least 35% of nickel to avoid instability and embrittlement through the formation of Laves phase on prolonged heating. Too much nickel, however, reduces the proof stress, and the nickel content must therefore not exceed 45 A particularly satisfactory combination of properties is exhibited by alloys that contain from 0.03 to 0.08%, and preferably 0.03 to 0.06% carbon, from 11 to 14% chromium, from 5.0 to 6.5% molybdenum, from 0.4 to 0.7% niobium, from 2.75 to 3.1% titanium, from 0.3 to 0.5% aluminium, from 40 to 45% nickel, and from 0.01 to 0.02% boron, the balance, apart from impurities, being iron.

To develop the best combination of proof stress and tensile ductility at elevated temperatures the alloys must be solution-heated and then aged in one or more stages. Advantageously solution-heating comprises heating at temperatures of from 1040 to 1080 C. for from 1 to 8 hours, preferably for from 2 to 4 hours. A single ageing treatment comprising heating at 700760 C. for at least 8 hours, preferably at 730 C. for 16 hours, is preferably employed. Alternatively the alloys may be aged in two stages, the first comprising heating at 740 to 780 C. for from 1 to 8 hours followed by heating at 675 to 720 C. for at least 8 hours in the second stage. One suitable ageing treatment of this kind comprises heating at 760 C. for 3 hours and then at 700 C. for 16 hours. Cooling after the solution-heating is preferably rapid, e.g. a quench in water, but cooling between ageing treatments may be conducted in air.

The present alloys, after solution-heating for 4 hours at 1060 C., water-quenching, and ageing for 16 hours at 730 C. and air-cooling, are generally characterised by a proof stress (0.2% offset) at 575 C. of at least 47 t.s.i.

stress by formation of the less desirable age-hardening precipitate Ni (Ti, Nb) in place of Ni (Ti, Al). In other alloys the presence of the former phase has been found to give low values of proof stress. As already mentioned increasing the aluminium content leads to a loss of tensile and proof strength, and the same effect results from replacement of titanium by aluminium at the same total content of titanium and aluminium. These effects are illustrated by the results of tests on extruded bar samples of alloys containing varying proportions of titanium and aluminium and otherwise having the same nominal composition as Alloy No. 1. The alloys were solution-heated for 2 hours at 1080 C., water-quenched, aged for 3 hours at 760, air-cooled, and finally aged for 16 hours at 700 C. and air-cooled. The results are set forth in Table II. Alloys Nos. 2 and 3 are in accordance with the invention, but alloys B and C are not.

TABLE II Tensile properties (room temp- Stress-rupture properties Percent erature Tensile properties (575 C.) 44 t.s.i./630 C.

0.1% P.S. I.S. E1. (per- 0.1% P.S. T.S. El. (per- Elongation Alloy N0. Ti Al (p.s.i.) (t.s.i.) cent) (t.s.i.) (t.s.i.) cent) Life (hrs) (percent) (longtons per square inch) and a tensile ductility of at Although the present invention has been described in least 20% elongation and a stress-rupture life of at least 50 hours under a stress of 44 t.s.i. at 630 C. They have excellent hot workability and can be deformed hot by rolling, forging, swaging and extrusion and can be deformed cold by rolling, drawing or swaging.

The improved tensile properties of the alloys of the invention are illustrated by the results of comparative tests in which two alloys A and 1 were prepared by air-melting and vacuum-refining followed by electroslag refining. Ingots cast from the alloys were forged to cheeses and then to rotor disc blanks 14 inches in diameter having a central punched-out bore 3 inches in diameter. After heattreatment to develop the optimum tensile properties, the disc blanks were sectioned and test-pieces were machined from similar positions adjacent and tangential to the central bore.

Alloy A had the nominal composition of Alloy 901 set forth above, and Alloy No. 1 was a preferred alloy according to the invention having the nominal composition, in percent by weight: carbon 0.04, chromium 12.5, nickel 42.5, molybdenum 5.75, titanium 3.0, aluminium 0.4, niobium 0.55, boron 0.015, silicon 0.2, manganese 0.l, balance iron. Alloy A was given a commercially recommended heat-treatment comprising solution-heating for 2 hours at 1095" C., water-quenching, and ageing for 2 hours at 775 C. and then for 24 hours at 705 C., with intermediate and final cooling in air. Alloy 1 was solutionheated for 4 hours at 1060 C., water-quenched, and then aged for 16 hours at 730 C. and air-cooled.

The results of tensile tests at 575 C. are set forth in Table 1.

The advantageous effect on the tensile properties of the co-presence of small amounts of niobium and aluminium is surprising, since from theoretical considerations it might have been expected that niobium would reduce the proof conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to Without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. An iron-base alloy characterised by high short-time elevated temperature strength consisting essentially of about 0.02 to 0.1% carbon, about 11 to 16% chromium, about 4 to 7% molybdenum, about 0.3 to 0.8% niobium, about 2.0 to 3.5% titanium, about 0.25 to 0.75% aluminium, about 35 to 45% nickel, about 0.003 to 0.02% boron, 0 to 0.1% zirconium, and the balance consisting essentially of iron.

2. An alloy in accordance with claim 1 that contains about 0.03 to 0.08% carbon.

3. An alloy in accordance with claim 2 that contains about 0.03 to 0.08% carbon, about 11 to 14% chromium, about 5.0 to 6.5% molybdenum, about 0.4 to 0.7% niobium, about 2.75 to 3.1% titanium, about 0.3 to 0.5% aluminium, about 40 to 45 nickel, about 0.01 to 0.02% boron, and the balance consisting essentially of iron.

4. An alloy in accordance with claim 3 in which the carbon content does not exceed 0.06%

5. A wrought gas turbine rotor disc made from the alloy set forth in claim 1.

6. A gas turbine rotor disc made from the alloy set forth in claim 3...

References Cited UNITED STATES PATENTS 2,813,788 11/1957 Skinner -128 3,065,067 11/1962 Aggen 75128.8 3,048,485 8/1962 Bieber 75-124 3,243,287 3/1966 Lillys 75-124 3,384,476 5/1968 Egnell 75-128 HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75128 

